I. Field of the Invention
This invention relates generally to pacemakers used for Bradyarrhythmia therapy, and more particularly, to control circuitry which may be disposed in the can of the pacemaker to sense the evoked ECG signal from the heart, or disposed in external equipment such as a Pacing System Analyzer.
II. Background of the Invention
Automatic capture is the feature of being able to ascertain whether or not a pace pulse from a pacemaker has in fact evoked a depolarization of the heart ventricle. When the ventricle depolarizes, the heart evokes a QRS complex which can be sensed and processed to verify that the heart depolarized, or to be processed such that therapeutic action by the pacemaker is facilitated. Many prior art approaches have focused upon altering the shape or polarity of the pacing pulse to the pacing electrodes to reduce polarization voltage upon them shortly after the pacing pulse terminates. This technique is known as biphasic or tripbasic pulse generation. Then, evoked response sensing can ensue much the same as normal intrinsic ECG sensing is done. The disadvantages of these approaches are that they end up using more energy than a single pulse, all the while to facilitate a feature of automatic capture-output regulation, which desires to lower device current drain by lowering the output amplitude closer to its minimum, known as the pacing threshold. Further, these approaches do not always adequately overcome the residual polarization on the bipolar electrode leads, which potentially can inhibit and degrade the sensing and analysis of the evoked ECG signal. Typically, potentials of 100 millivolts or more can remain across the bipolar electrodes after a depolarization of the ventricle. In contrast, the normal evoked ECG response can be as low as 5 millivolts. Thus, the ability to sense a low voltage ECG signal by using the bipolar lead electrodes can be difficult when the electrodes remain polarized.
U.S. Pat. No. 4,858,610 to Callaghan et al., teaches an apparatus and method of detection of cardiac evoked potentials. This invention teaches applying a stimulation pulse in a unipolar mode to the tip electrode, wherein the ring electrode is used to sense the evoked ECG signal, also in a unipolar mode. The evoked ECG signal is sensed by the ring electrode during a predetermined window after generation of the pacing pulse. A charge dump circuit is taught for discharging the polarization potential of the tip electrode prior to the window for sensing the evoked ECG signal. Accordingly, signal interference caused by residual polarization of the first electrode is reduced. However, this device still requires biphasic and tripbasic pulse generation, which is more complicated and requires more energy than desired. This technique reduces the useful life of the internal pacemaker battery.